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New submitter Doug Otto sends word that researchers working on the ALPHA experiment at CERN are trying to figure out whether antimatter interacts with gravity in the same way that normal matter does. The ALPHA experiment wasn't designed to test for this, but they realized part of it — an antihydrogen trap — is suitable to collect some data. Their preliminary results: uncertain, but they can't rule it out. From the article:
"Antihydrogen provides a particularly useful means of testing gravitational effects on antimatter, as it's electrically neutral. Gravity is by far the weakest force in nature, so it's very easy for its effects to be swamped by other interactions. Even with neutral particles or atoms, the antimatter must be moving slowly enough to perform measurements. And slow rates of motion increase the likelihood of encountering matter particles, leading to mutual annihilation and an end to the experiment. However, it's a challenge to maintain any antihydrogen long enough to perform meaningful experiments on it, regardless of its speed. ... The authors of the current study realized that [antiatoms trapped in ALPHA] eventually escaped or were released from this magnetic trap. At that point, they were momentarily in free-fall, experiencing no force other than gravity. The detectors on the outside of ALPHA could then determine if the antihydrogen was rising or falling under gravity's influence, and whether the magnitude of the force was equivalent to the effect on matter."

Maybe our universe is a 'matter bubble' in a 'sea of anti-matter'. WE are the anti-matter.

To us, our normal matter is so common but that's only because we're sitting right smack in the middle of it. That would explain the repelling forces and show why dark matter could exist outside the bounds of the observable universe.

Actually, I'm pretty sure a lot of them would have the opposite reactions. When the Higgs Boson was finally found, a lot of physicists were actually disappointed because it meant there wasn't really much in the way of new physics to be discovered.

we already know antimatter doesn't have "negative mass" in that sense, it responds with expected inertia to acceleration by electromagnetic forces. we already know the yield of annhilation too (relativistic mass is positive). question is just of response to gravitational field of normal matter, which way the force vector points.

Based on what we currently know, we would expect that the only significant force acting on a piece of falling antimatter is gravity; by the equivalence principle, this should make antimatter fall with the same acceleration as ordinary matter. However, some theories predict new, as yet unseen forces: these forces would make antimatter fall differently than matter. But in these theories, antimatter always falls slightly faster than matter; antimatter never falls up. This is because the only force that would treat matter and antimatter differently would be a vector force (mediated by the hypothetical gravivector boson). Vector forces (like electromagnetism) repel likes and attract opposites, so a gravivector force would pull antimatter down toward the matter-dominated Earth, while giving matter a slight upward push.

Inertial mass and gravitational mass are observed - for normal matter - to be exactly equivalent. There's no actual reason they should be though, since they're the product of very different interactions

Well, if you believe General Relativity, they darn well better be equivalent. In fact, Einstein took the Equivalence Principle as one if his starting points when developing GR. If the Equivalence Principle fails (which it must if anti-matter falls up), then they will have disproven Einstein's theory, which would be very big news, indeed.

Which would hence be the value of a test that it is in fact enforced. Again: we can only assume it's true because the laws we know work in other cases assume it's true. But there's no implicit reason to think that we aren't simply observing a whole lot of local cases where some higher principle is simplifying to General Relativity, or where at the fringes there's a small correcting constant which isn't significant in most normal situations.

This type of measurement is where new physics comes from - it's why there's people who have been measuring alpha to ever greater precision, even though we've no reason to think it'll deviate if current theory is a complete explanation.

Faster than light travel is only impossible when you have a net positive mass. If your mass is net zero, (meaning in your magnetic grip you hold matter and antimatter in the same functional unit but not touching each other (two magnetic bottles), then you could travel faster than the speed of light.

It would seem that antimatter could only fall up, if there was some way to distinguish gravitational and inertial mass. From my experience of how electrons and positrons were accelerated at SLAC, their inertial mass was identical. The only difference between them was their charge.

This is why it is important to conduct the experiment to see if the gravitational and inertial mass of antimatter are the same. Sure, we know that they're the same thing for ordinary matter and that antimatter and matter have the same inertial mass, but the effect hasn't been properly studied for antimatter (because that's a furiously difficult experiment). It could be that gravitational and inertial mass are the same for AM; that would be the most likely expected case, and we wouldn't learn that much about new physics if that's true. But we haven't checked, and so we must do so to make sure. After all, if they were different that would be a really important fact about the universe that we are currently unaware of. (It would be far more important than finding the Higgs boson.)

Let the experiment be done. Let us find out if the universe is even stranger than we thought it was. It's this sort of thing that a fundamental physics lab should study.